Femoral head resurfacing apparatus and methods

ABSTRACT

A femoral head resurfacing apparatus includes a cap of hemispherical shape and stabilization structure (e.g., one or more non-shear fixation bars) that inserts into slots cut into the femoral head, so that the cap remains substantially immovable other than due northerly. A method of resurfacing the femoral head of a hip joint includes removing longitudinal slots or holes of bone in the femoral head, and attaching a hemispherical cap to the head, the cap having non-shear fixation bars or other stabilizing structure for mating engagement with the slots or holes.

RELATED APPLICATIONS

This application is a continuation of commonly-owned and copending U.S.patent application Ser. No. 10/937,047, filed 9 Sep. 2004, which is acontinuation-in-part of U.S. patent application Ser. No. 10/200,239,filed 22 Jul. 2002, which claims priority to U.S. Provisional patentApplication Ser. No. 60/307,244, entitled “Total Hip Joint ReplacementProsthesis” filed 23 Jul. 2001. Each of the above-identifiedapplications is incorporated herein by reference.

BACKGROUND OF THE INVENTION

Various partial or complete replacements of the hip joint have beenproposed and used since the early 1900's. Most of the procedures ormethods involved material problems that led to joint loosening andconsequential failure.

One widely used total hip replacement removes the femoral head andinserts a stem into the upper end of the femur, where it is fixed eitherby cement or by bone growth into a porous coating. A small metal ball,replacing the patient's femoral ball, is affixed to the stem. Thistechnique requires massive bone removal and results in extreme loadingof force in a leveraging action from the top of the femoral ball to alower part of the stem. Over a few years, force applied by vigorous andmostly younger patients can cause the stem to loosen, resulting infailure, pain and need for extensive and expensive revisions.

When a patient has enough bone stock, a resurfacing approach may beused. In one example, the femoral head is resurfaced with a singularfixation approach that uses either cementing or bone ingrowth into aporous surface or surfacing material. However, with the singularfixation approach, the probability of failure remains high because thereis no mechanism to back up, reinforce and absorb large downward forcesand subsequent reactionary forces applied through vigorous activity.

Accordingly, after a hip joint replacement utilizing prior arttechniques, slight movements (up to about 0.08 mm per year) of areplacement shell relative to a femoral ball may occur and are deemed“acceptable migration.” Single point fixation methods typically mitigatethis problem with respect to only one degree of freedom in movement; forexample, a centering post can stop a cap or shell from migratinglongitudinally, but does not prevent it from migrating equatorially. Thepolar north direction is not a problem, in that anatomically most forceapplied to a femoral head is driving the shell on, versus pulling itoff. Cement is capable of preventing loosening in the polar northdirection. However, the equatorial direction receives a larger shearstress. Elasticity of typical cements allows “acceptable migration” dueto the application of force, but when a cement's coefficient ofelasticity is reached, the cement will fail with significant loosening,requiring a revision. Other factors that can contribute to cementfailure are:

-   1. Variance in quality of manufactured cement—Quality control-   2. Variance in bone composition from one individual to another,    which could mean a difference in adhesion-   3. Variance in operation technique.-   4. Too much or too little cement applied.-   5. Foreign material getting into the cement.-   6. Different levels of patient activity may exert significantly    higher amounts of force on the shell, causing varying degrees of    migration.

SUMMARY OF INVENTION

Multiple embodiments of femoral resurfacing apparatus (including afemoral cap or shell, hereinafter “cap”) and fixation methods aredisclosed that may require less bone removal than prior art caps andmethods; they may also maintain the naturally occurring femoral headshape, to accept and evenly distribute applied and reactionary forces tothe femur and lessen the potential for shearing and loosening of thecap. Methods of fixation may employ different approaches to absorb aportion of the applied and reactionary shearing forces generated byhighly active patients, thus reducing the possibility of loosening (aprincipal cause of hip joint replacement failure).

In one method, mechanical fixation uses internal metal non-shear bars(e.g., three non-shear bars) that are 5-8 mm wide and about 4 mm deep;slots cut into the femoral ball accommodate the non-shear bars. Thenon-shear bars are an integral part of a hemispherical femoral cap, andreinforce and strengthen the cap to withstand shear forces many timesthose which are generated by vigorous activity. Therefore, this methodof fixation prevents lateral and longitudinal rotation of the cap, whileutilizing only two components that can shear or loosen (e.g., the capand the bone). Accordingly, the cap can separate from the femoral ballin only one direction: due northerly, that is, straight off the femoralball without rotation. This direction of separation may be mechanicallyprevented by metal spring loaded absorption fixators (SLAFs) that screwinto the ends of the non-shear bars at their hemispherical ends. TheSLAFs extend downward, for example 10-15 mm, whereupon they areseparated from the side of the femoral ball by approximately 3 mm. Eachspring loaded absorption fixator (SLAF) may be screwed into the femoralball, pulling the tab against the side of the lower femoral ball. A loadfor each SLAF may be selected from a range of 30# to 130# to (1) providea southerly retention force equal to, for example, one third of thepatient's body weight on the cap, and (2) absorb a portion of theapplied and reactionary forces received by the femoral ball similar tonormal bone flexation. Additionally, the screws may be prevented frombacking out by seating them deep enough within the SLAF so that a smallflat metal anti-back out tab can be inserted into a slot runningparallel to the head surface of each screw. Once this anti-back out tabis inserted into the slot, it may drop down into a position lower thanthe slot itself, so that it can not come out without the use of specialremoval tools. The anti-back out tab sits on top of the head of thescrew to prevent back out of the screw. In addition, threads of the bonescrews may be notched so that, once in place, bone will grow into thenotches, providing a back up anti-back out screw fixation method. Thelower or southerly ends of the SLAFs may also be coated withhydroxyapatite porocast, or other materials, to create fixation via boneingrowth.

Another method of preventing a separation or loosening of a cap in anortherly or straight off non-rotational direction is by bone-to-bonegrowth through gear tooth shaped teeth cast into the bottom surface ofthe non-shear bars starting northerly about 8 mm from the hemisphericalend of each bar and running about 15 mm down each non-shear bar. Eachtooth is about 2 mm deep and 2 mm wide. A coating with hydroxyapatiteporocast reduces the depth and width to about 1 mm, providing forsubstantial purchase through bone growth into the teeth from threedirections.

A third method of fixation may employ a coating of the interior of theshell with hydroxyapatite porocast, or similar bone ingrowth fixationmaterial, to prevent separation or loosening of the cap in alldirections.

In one non-limiting example of a total hip joint replacement operation,access to the hip joint is achieved through separation of thetrochanter, which protrudes from the upper portion of the upper leg bone(the femur) from the rest of the leg, together with dislocation of thehip joint itself. After dislocation, the femoral ball is reamed down toaccommodate a hemispherical, hollow cap that has approximately the sameoutside diameter as the patient's (pre-ream) femoral ball. Uponcompletion of the reaming process, the femoral ball remainssubstantially hemispherical, with its diameter and surface reduced by 4mm to 5 mm in order to accommodate the metal alloy femoral cap.Additionally, slots are cut longitudinally an equal distance apart inthe femoral ball, to accommodate the non-shear bars within the cap thatrun perpendicular to the equatorial edge or lip of the cap north/southfrom its polar orientation a distance of 25 mm to 30 mm. With thenon-shear bars, the cap cannot separate from the femoral ball throughrotation, laterally or longitudinally. The cap is also mechanicallyfixed by SLAFs screwed to the internal longitudinal cap's non-shearbars, and subsequently to the femoral ball, by full thread cancellousscrews about 4 mm long; thus securing the cap so that it can notseparate from the femoral ball by traveling in a non-rotationaldirection due north. Additionally, the underside of the lower end of theSLAFs may be coated with hydroxyapatite porocast to provide a secondfixation method for the SLAFs, through bone growth into a hydroxyapatiteporocast surface. Additionally, the internal longitudinal non-shearbars' lower surfaces, which face the bottom of the slots, may includegear teeth that allow bone to grow more substantially into the teeth,providing a third method of fixing the cap to the femoral ball, so thatthe cap can not move or separate from the femoral ball in a straightcephalad northerly direction. Further fixation may be achieved bycoating the inside surface of the cap between the non-shear bars withhydroxyapatite porocast, or similar materials, to fixate the cap to thefemoral head by bone ingrowth.

In one embodiment, a femoral head resurfacing apparatus includes (a) acap with a substantially hemispherical shape of substantially uniformthickness, and (b) one or more non-shear fixation bars that insert intorespective longitudinal slots cut into the femoral head, so that the capremains substantially immovable other than due northerly. The non-shearfixation bars may include a plurality of gear shaped teeth to encouragebone ingrowth between the teeth.

In another embodiment, a femoral head resurfacing apparatus includes (a)a chromium-cobalt-molybdenum alloy cap having a substantiallyhemispherical shape of substantially uniform thickness, and (b) aplurality of non-shear fixation bars for insertion into respectivelongitudinal slots cut into the femoral head, so that the cap remainssubstantially immovable other than due northerly. The cap may interfacewith a metal cup socket maintaining a clearance tolerance ofapproximately 0.01 mm.

In another embodiment, a femoral head resurfacing apparatus includes (a)a cap having a substantially hemispherical shape of substantiallyuniform thickness, and (b) a plurality of non-shear fixation bars forinsertion into respective longitudinal slots cut into the femoral head,so that the cap remains substantially immovable other than duenortherly. The cap may be fixed by a plurality of metal spring loadedabsorption fixators, each fixator being screwed into the cap and femoralhead, to prevent loosening of the cap.

In another embodiment, a method of resurfacing the femoral head of a hipjoint includes removing longitudinal slots of bone in the femoral head,and attaching a hemispherical cap to the head. The cap has non-shearfixation bars that engage with the slots.

In another embodiment, a hollow femoral head resurfacing cap includesnon-shear bars for mating engagement with longitudinal slots within afemoral head, and spring-loaded absorption fixators for attaching thecap to the head.

In another embodiment, a method for capping a femoral head with a shellincludes inserting a centering device in a centering hole of the femoralhead, drilling holes about the centering hole, and inserting fin-likedowels, which are a part of the centering hole post, into the holes toprevent migration of the shell. The three hole diameters may vary insize from about 3 mm to 12 mm; their depth may be about 6 mm.

In another embodiment, a femoral head resurfacing apparatus includes (a)a cap of hemispherical shape, (b) a center post for insertion into acenter hole in the femoral head; and (c) stabilization structure forinsertion into one or more corresponding holes in the femoral head, sothat the cap remains substantially immovable other than due northerly

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an upper end of a femur, a pelvis with a metal actabularcup, and a femoral head resurfacing apparatus, in accord with oneembodiment.

FIG. 2 is a cross-sectional view of a femoral head with arthriticcalcifications.

FIG. 3 is a cross-sectional view of a femoral head, after down sizereaming, and a femoral head resurfacing cap in accord with oneembodiment.

FIG. 4 is an internal view of a template device.

FIG. 5 is a side/bottom view of a depth gauge device.

FIG. 6 shows a femoral head with three slots.

FIG. 7 is an internal view of a femoral head resurfacing cap showingthree non-shear fixation bars, in accord with one embodiment.

FIG. 8 shows the femoral head resurfacing cap of FIG. 7 in place,capping a femoral head.

FIG. 9 shows a spring-loaded absorption fixator (SLAF) screwed into anequatorial end of a non-shear fixation bar, in accord with oneembodiment.

FIG. 10 shows further details of the SLAF of FIG. 9.

FIG. 11 shows the SLAF of FIG. 9 prior to being screwed onto a femoralhead.

FIG. 12 is a perspective view of two anti-backout locking mechanismsthat are integrated within a SLAF, in accord with one embodiment.

FIG. 13 is a side view of the SLAF of FIG. 12 showing a slot into whichthe anti-backout tab of FIG. 12 may be inserted to prevent back-out of abone screw.

FIGS. 14A and 14B are top and side views of the anti-backout tab of FIG.12.

FIGS. 15A and 15B are enlarged side and end views of a southerly end ofthe SLAF of FIG. 12.

FIG. 16 shows how an anti-backout tab may be removed by a surgeon, toremove a bone screw.

FIG. 17 shows one anti-backout 3.5 mm cancellous screw.

FIGS. 18A and 18B are a side cross-sectional view and a top view,respectively, of one femoral head resurfacing cap embodiment.

FIG. 19 shows a template for a central hole and side holes that may bemade in a femoral head to accommodate the femoral head resurfacing capof FIGS. 18A and 18B.

FIG. 20A and 20B show, respectively, a perspective view and an end viewof a femoral head cap that has a stem with a lengthwise groove.

FIG. 20C and 20D show, respectively, a dowel for use with the femoralhead cap of FIG. 20A and 20B, and an end view of the femoral head cap ofFIG. 20A and 20B with the dowel.

FIGS. 21A and 21B show an end view and a side view, respectively, of afemoral head with holes drilled to accept the stem of FIGS. 20A, 20B and20D and the dowel of FIGS. 20C and 20D.

DETAILED DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an upper end of a large upper leg bone, namely a femur 10,a pelvis 12 with a metal actabular cup 16, and a femoral headresurfacing apparatus 9 that includes a femoral head resurfacing cap 14,in accord with one embodiment. A trochanter 18 is a large protuberanceat the upper end of femur 10. Many major muscles which secure a femoralhead 20 into a socket of the pelvis 12 attach to trochanter 18;accordingly, separation of trochanter 18 from femur 10 permits adislocation of the hip joint fairly easily, giving good access todiseased areas of an arthritic or otherwise diseased hip joint.

FIG. 2 is a cross-sectional view of a femoral head 20 with arthriticcalcifications 22 and an outside diameter (O.D.) 24, prior to reamingoff arthritic calcifications 22 and reducing the femoral head 20 toaccept femoral head resurfacing cap 14, as shown in FIG. 3. Harder outerbone structure 26 of femoral head 20 is shown without cross hatching,and a strong calcificous inner portion 28 of femoral head 20 is shownwith cross hatching.

FIG. 3 is a cross-sectional view of a femoral head 20, after down sizereaming, and a femoral head resurfacing cap 14 in accord with oneembodiment. Reaming reduces O.D. 24 by about 6 mm to form a modifiedO.D. 30, without changing the normal shape of femoral head 20. An innersurface of cap 14 may be coated with very small protuberances andindentations 32, such as a surface formed with a hydroxyapatite porocastprocess (to accept bone ingrowth), fixing cap 14 to femoral head 20. Cap14 has an outside diameter 34 that is equal or close to the originalsize 24 of femoral head 20 (prior to reaming). Hard outer bone structure26 is mostly removed from the top and sides of femoral head 20 duringthe reaming and shaping process, as shown in FIG. 3. Additionally, FIG.3 shows one of three non-shear fixation bars 36 with gear shaped teeth38 having a depth of about 2 mm and gaps between teeth of about 2 mm.Non-shear fixation bars 36 run within cap 14 from an equatorial plane oredge 40 northward by a distance 42 of about 18 mm, depending on the sizeof the cap 14 used. Each non-shear fixation bar 36 extends radiallyinward from an inner surface of the cap 36 by a distance 44 of about 4mm, and is 5 mm to 8 mm in width (see FIG. 7).

FIG. 4 is an internal view of a template device 46 having an insidediameter slightly larger than the reamed femoral ball outside diameter30, FIG. 3. Once femoral head 20 has been reamed and downsized, template46 may be placed over femoral head 20. Slots 48 in template device 46are traced and marked on femoral head 20 at the locations at which thesurgeon desires to cut slots for non-shear fixation bars 36 shown inFIGS. 3, 7 and 8. In one embodiment, slots 48 have a length 50 of 15 mmto 18 mm, and a width 52 of about 9 mm.

FIG. 5 is a side/bottom view of a depth gauge device 54 that a surgeonmay use to detennine a correct depth to cut the traces of slots 48 onfemoral head 20, so that the femoral head may receive the threenon-shear fixation bars 36 located on the inner surface of the femoralcap 14.

FIG. 6 shows a femoral head 20 with three slots 56 cut in positionsdetermined by a surgeon using template device 46, FIG. 4, to the depthindicated by depth gauge device 54, to accept the three non-shear bars36 located on the inner surface of the cap 14 as shown in FIGS. 3, 7 and8.

FIG. 7 is an internal view of a femoral head resurfacing cap 14 showingthree non-shear fixation bars 36, in accord with one embodiment. Eachnon-shear fixation bar 36 has a depth 44 of about 4 mm, a width 58 of,for example, 5 mm to 8 mm, and a length 42 of about 18 mm. Eachnon-shear fixation bar 36 has, for example, three teeth 38 in an innersurface 60 of each bar, starting at a distance 62 of about 5 mm from afront equatorial end 70 of each non-shear fixation bar 36; each tooth 38has a flat top width 64 of about 1 mm and an interior tooth depth 66 ofabout 2 mm. Gaps between the tops of adjacent teeth 38 are about 2 mm,and an interior angle or slope of each tooth 38 is, for example, 45degrees. Teeth 38 may be coated with hydroxyapatite porocast, for boneingrowth fixation, reducing the width and depth of each tooth by about 1mm and increasing the depth of the non-shear fixation bars 36 by about 1mm, for a total depth of about 5 mm. In addition, the sides of eachnon-shear fixation bar 36 may also be coated with hydroxyapatiteporocast for additional bone ingrowth fixation, increasing the width ofeach non-shear fixation bar by the thickness of the coating(approximately 1 mm) for a total width ranging from 6 mm to 9 mm. Theequatorial end 70 of each non-shear fixation bar 36 may be tapped toreceive a machine screw 128, which may for example accept an Allenwrench. The thread and material design of machine screw 128 may bespecified to withstand a shear force of about 1000 pounds.

FIG. 8 shows femoral head resurfacing cap 14 on femoral head 20.

FIG. 9 shows a spring loaded absorption fixator (SLAF) 130 screwed intoequatorial end 70 (hidden in this view) of a non-shear fixation bar 36,in accord with one embodiment.

FIG. 10 shows, in further detail, SLAF 130 with a northerly end 132 anda southerly end 134. A machine screw 128 attaches a northerly end 132 ofeach SLAF 130 to an equatorial end 70 of non-shear bar 36, as shown inFIGS. 8 and 11. A bone screw 140 that attaches SLAF 130 to a femur 10 isalso shown.

FIG. 11 shows one SLAF 130 prior to being screwed onto femoral head 20.Each SLAF 130 incorporates a bend of about 40 degrees, so that afterbeing screwed into the cap 14, the opposite end is a distance 136 ofabout 3-4 mm off of femoral head 20. After bone screw 140 screwssoutherly end 134 onto femoral head 20, compression of SLAF 130 exerts aforce 137 in a southerly direction on cap 14. Force 137 may be from onethird to the total weight of the patient, depending on a stiffness ofSLAF 130, which is selected by the surgeon. The stiffness of each SLAF130 may be selected from a range of 30 lbs. to 120 lbs. in 10 lb.increments, in order to provide a total southerly caudal retention force137 on cap 14, and also to absorb a portion of the applied andreactionary forces received by the femoral head, similar to theflexation of the bone itself. SLAFs 130 may thus provide a continuoushold down or fixation force to the cap 14, while flexing like femur 10to absorb applied downward and reverse reactionary forces, to reducelikelihood of shearing or breaking. Additionally, a southerly undersidearea 138 of SLAF 130 may be coated with hydroxyapatite porocast foradditional bone ingrowth fixation.

FIG. 12 is a perspective view of two anti-backout locking (ABL)mechanisms 150 that are integrated within SLAF 130, to prevent bonescrews 140 and machine screws 128 from loosening or backing out of thefemur 10 or non-shear bars 36 respectively, in accord with oneembodiment. In northerly end 132 of SLAF 130, a hole 152 receives amachine screw 128 (not shown); in southerly end 134, a hole 154 receivesa bone screw 140. In each ABL 150 is a hollow cavity or slot 156, 156′about 1.5 mm wide (also shown in FIG. 15) to receive an anti-backout tab162. In northerly end 132, end face 158 of SLAF 130 has a slot 156,while in southerly end 134, side face 160 has a slot 156′. Southerly end134 is shown split along slot 156′ in FIG. 13 so that the internalstructure of southerly end 134 may be viewed. Each of slots 156, 156′also includes a recess 160 for use with a stylus tool (as shown in FIG.17).

FIG. 13 is a side view of SLAF 130 showing slot 156′ into which ananti-backout tab 162 may be inserted to prevent bone screw 140 frombacking out. Recess 160, machine screw 128 and bone screw 140 are alsoshown.

FIG. 14A and 14B are top and side views of anti-backout tab 162, whichhas a thickness 164 of about 1.4 mm and is made of stainless springsteel. A proximal end 166 of anti-backout tab 162 has a downward bend170 of approximately 10 degrees relative to a distal end 168, as shown.Anti-backout tab 162 also has a hole 172, as shown.

When inserted into slot 156, anti-backout tab 162 first straightens,then snaps back to its original bent shape when proximal end 166 movespast a lip 182 (see FIG. 16), thus locking the respective machine screw128 or bone screw 140 in place. SLAF 130 may be made of stainless springsteel and be configured so that when northerly end 132 is attached to anon-shear fixator bar 36 by a machine screw 128, southerly end 134 iselevated by 3-4 mm (distance 136 shown in FIG. 11) off of the lower endof the femoral head 20. In this manner, when screwed down with bonescrew 140, SLAF 130 delivers a southerly fixation force to cap 14.

FIG. 15 is an enlarged side and end view of southerly end 134 of SLAF130. Inside slot 156 is a lip 182 adapted to hold anti-backout tab 162.When fully inserted into slot 156 past lip 182, bend 170 snaps proximalend 166 of anti-backout tab 162 down so that anti-backout tab 162 doesnot work free of ABL 150, thus preventing bone screw 140 from backingout.

FIG. 16 shows how anti-backout tab 162 may be removed by a surgeon forremoval of a bone screw 140. A point 186 of a stylus 188 is insertedinto groove 160 at the bottom of slot 156′ of anti-backout tab 162 (alsosee FIGS. 9, 13 and 15). Groove 160 is deep enough to allow point 186 ofstylus to get underneath anti-backout tab 162, so that pressure may beapplied to straighten bend 170, enabling anti-backout tab 162 to movepast lip 182. At the same time, a second stylus (not shown) may beinserted into hole 172 in anti-backout tab 162, to push anti-backout tab162 out through slot 156′, enabling removal of bone screw 140. Anidentical procedure may be used on a northerly end 132 of SLAF 130 toremove a machine screw 128.

FIG. 17 shows one anti-backout 3.5 mm cancellous screw 190. Screw 190has a length 192 and threads 193, as shown. Exemplary dimensions ofscrew 190 may include a thread width 194 of 3.5 mm, a head diameter 196of 6.0 mm, and a hex socket of 2.5 mm. Threads 193 have three notches(not shown), each notch about 0.025 mm deep and 0.03 mm wide, runninglengthwise down about 70% of length 192, and each notch spaced an equaldistance from adjacent notches about the circumference of screw 190.Once cancellous screw 190 is in place, bone growth can invade thesenotches, thus locking screw 190 in place, providing anti-backoutcapability.

FIGS. 18A and 18B are a side cross-sectional view and a top view,respectively, of another femoral head resurfacing cap 200. Sidecross-sectional view FIG. 18A is taken through a plane indicated bydashed line 18A-18A in FIG. 18B. A centering post 202 and one or morerods 204 are affixed to an inner surface 206 of shell 200. Rods 204 formstabilizing structure that may be inserted into corresponding holes orslots in a femoral head 20 (not shown) to stop migration or loosening ofthe shell in an equatorial or lateral direction. Once seated, shell 200does not rotate equatorially without shearing femoral head 20.Additionally, rods 204 provide resistance to migration. Inner surface206 of shell 200 may be coated with very small protuberances andindentations, such as a surface formed with hydroxyapatite porocast, toaccept bone ingrowth and help fix cap 200 to femoral head 20.

FIG. 19 shows a template 250 for a central hole 252 and side holes 254that may be made in femoral head 20 (not shown) to accommodate femoralhead resurfacing cap 200. Central hole 252 may be sized to match adiameter of centering post 202 of cap 200, and side holes 254 may besized to match a diameter of rods 204. A surgeon may use template 250 tomark sites on a femoral head for drilling, or may drill holes withtemplate 250 in place.

Although FIGS. 18A, 18B and 19 show stabilizing structure of three rods204, and a template 250 for holes to accommodate three rods 204, adifferent number or type of structure may be used. For example, more orfewer rods 204 may be used. Furthermore, a stabilizing structure may beof another shape, for example, it may include members that are round,square, or triangular. A stabilizing structure may abut centering post202, or may be affixed to inner surface 206 at locations which are notadjacent to the centering post. Alternatively, a stabilizing structuremay be formed monolithically with centering post 202; for example, rods204 may be fins or fin-like dowels extending from centering post 202. Atemplate (i.e., like template 250) may be configured to locate sites forcutting holes, slots or other openings in a femoral head to accommodatethe number, locations, and shapes of rods 204.

FIG. 20A and 20B show, respectively, a perspective view and an end viewof a femoral head cap 300 that has a stem 302 with a lengthwise groove304. Stem 302 may be approximately 3 to 4 centimeters long, and may becircular in cross-section with a diameter 303, except at groove 304.Groove 304 is a semi-circular recess within stem 302.

FIG. 20C and 20D show, respectively, a dowel 306 for use with femoralhead cap 300 and an end view of femoral head cap 300 with dowel 306.Dowel 306 is shorter than stem 302, and is adapted to fit snugly intogroove 304, as shown, to form a stabilizing structure. A diameter 307 ofdowel 306 may be, for example, 10% to 50% of diameter 303 of stem 302.

Although FIGS. 20A, 20B and 20D show stabilizing structure of one dowel306, and a femoral head 310 with a hole to accommodate dowel 306, morethan one dowel 306 may be used, with additional stem grooves (i.e., likegroove 304) and femoral head holes (i.e., like hole 314) to accommodateeach dowel 306. Multiple dowels 306 may be identically sized, or may bedifferent from one another, with corresponding adjustments to the stemgrooves and femoral head holes.

FIG. 21A and 21B show an end view and a side view, respectively, of afemoral head 310 with holes 312, 314 drilled to accept stem 302 anddowel 306. Hole 312 has a sufficient diameter and depth to accept stem302, and hole 314 has a sufficient depth and diameter to accept stem304. To install femoral head cap 300, a surgeon (optionally using atemplate with appropriately located and sized holes) drills holes 312and 314. The surgeon then inserts dowel 306 into groove 304, and slidesstem 302 and dowel 306 into holes 312 and 314, respectively. Hole 314holds dowel 306 in place, and dowel 306 in turn prevents stem 302 andcap 300 from rotating with respect to femoral head 310. Hole 312 holdsstem 302 in place and prevents cap 300 from migrating.

The changes described above, and others, may be made in the femoral headresurfacing apparatus and methods described herein without departingfrom the scope hereof. It should thus be noted that the matter containedin the above description or shown in the accompanying drawings should beinterpreted as illustrative and not in a limiting sense. The followingclaims are intended to cover all generic and specific features describedherein, as well as all statements of the scope of the present method andsystem, which, as a matter of language, might be said to fall therebetween.

1. Femoral head resurfacing apparatus, comprising a cap of substantiallyhemispherical shape; a center post for insertion into a center hole inthe femoral head, the center post forming one or more grooves; andstabilizing structure for insertion into one or more corresponding holesin the femoral head, the stabilizing structure comprising one or moredowels, each dowel configured to fit into a corresponding groove;wherein the cap remains substantially immovable other than duenortherly.
 2. Apparatus of claim 1, an inner surface of the capcomprising hydroxyapatite porocast to promote bone ingrowth-penetrationinto the cap and facilitate fixation of the cap to the femoral head. 3.Apparatus of claim 1, the cap being sized to minimize bone removal fromthe femoral head.
 4. A method for capping a femoral head with a shell,comprising the steps of: drilling a central hole in the femoral head;forming one or more additional openings in the femoral head; inserting acentering post of the shell into the central hole; and insertingstabilizing structure, which is formed monolithically with the centeringpost, into the one or more additional openings to prevent migration ofthe shell.
 5. Femoral head resurfacing apparatus, comprising: a cap ofsubstantially hemispherical shape; a centering post for insertion into acentral hole in the femoral head; and fins, extending from the centeringpost, for insertion into one or more corresponding openings in thefemoral head; wherein the cap remains substantially immovable other thandue northerly.
 6. Apparatus of claim 1, the center post being 3 to 4centimeters long.
 7. Apparatus of claim 1, a diameter of the one or moredowels being 10% to 50% of a diameter of the center post.
 8. Apparatusof claim 1, the one or more dowels comprising a plurality of dowels, theone or more grooves comprising a plurality of grooves such that eachdowel corresponds to a selected one of the grooves.
 9. Apparatus ofclaim 8, wherein at least one of the dowels is sized differently than atleast one other of the dowels.
 10. Method of claim 4, further comprisinga step of down size reaming the femoral head so that the femoral headfits within the shell.
 11. Method of claim 4, wherein at least one ofthe steps of drilling and forming comprises utilizing a template. 12.Apparatus of claim 5, one or more of an inner surface of the cap, asurface of the centering post and a surface of the fins comprisinghydroxyapatite porocast.
 13. A method for capping a femoral head with ashell, comprising the steps of: drilling a central hole in the femoralhead; drilling one or more secondary holes in the femoral head such thateach of the secondary holes adjoins the central hole; inserting, foreach of the one or more secondary holes, a corresponding dowel into acorresponding groove formed by a centering post of the shell; insertingthe centering post into the central hole; and inserting eachcorresponding dowel into each of the one or more secondary holes, toprevent migration of the shell.
 14. Method of claim 13, furthercomprising a step of down size reaming the femoral head so that thefemoral head fits within the shell.
 15. Method of claim 13, wherein atleast one of the steps of drilling the central hole and drilling one ormore secondary holes comprises utilizing a template.